Monolithic brushless DC motors are increasingly utilized as fan motors is various computer applications. Pulse width modulation drive circuits are commonly used in conjunction with control circuits to control the speed of such fan motors.
In some applications, Hall effect type sensors and amplifiers are utilized to generate signals to determine the rotational position and speed of the motor.
One problem that occurs when utilizing Hall effect type sensors in conjunction with PWM driven motors is that the leading and trailing edges of each pulse of the PWM signals induces a noise spike in the Hall sensor outputs.